Joining the cast of the beloved teen soap in 1994 after Doherty's scandal-plagued departure, Thiessen, 38, and Garth quickly became BFFs off-camera -- with Garth even tapping Thiessen to be a bridesmaid at her wedding.

But they've since had a falling out, sources tell Us Weekly.

During a 2009 party, a guest recalls, a stone-faced Garth wouldn't even look at the "White Collar" actress when Thiessen said hello.